In a laser scanner, a light beam generated by a laser periodically sweeps over a monitoring area by means of a deflecting unit. The light is remitted by objects in the monitoring area and evaluated in the laser scanner. The angular position of the object is detected from the angular position of the deflecting unit, and additionally the distance of the object from the laser scanner is detected from the light time of flight using the speed of light. For conventional laser scanners, two basic principles are known for determining the light time of flight. In phase-based methods, the continuous transmission light is modulated, and the phase shift of the received light with respect to the transmitted light is evaluated. In pulse-based methods or pulse time of flight methods, the transmitter is operated in a single pulse mode with comparatively high pulse energies, and the laser scanner measures object distances on the basis of the time of flight between the transmission and reception of a single light pulse. In a pulse averaging method known for example from EP 2 469 296 B1, a plurality of individual pulses are transmitted for a measurement, and the reception pulses are statistically evaluated.
With the angle and distance data, the location of an object in the monitoring area is detected in two-dimensional polar coordinates. This can be used to determine object positions or their contour. The third spatial coordinate can also be detected by a relative movement in the transverse direction, for example by an additional degree of freedom of the deflection unit in the laser scanner, or in that the object is moved relative to the laser scanner. Thus, three-dimensional contours can also be measured.
Laser scanners are not only used for general measurement tasks, but also in safety technology to monitor a source of danger, for example a dangerous machine. Such a safety laser scanner is known from DE 43 40 756 A1. A protection field is monitored which must not be entered by personnel during operation of the machine. In case the laser scanner detects an inadmissible protection field intrusion, like a leg of a person, it triggers an emergency stop of the machine. Other intrusions into the protection field, for example by stationary machine parts, can be taught in advance as permissible. Often, warning fields are arranged in front of the protection fields, where intrusions at first only cause a warning, in order to prevent the protection field intrusion and thus the emergency stop in advance, therefore increasing availability of the system. Safety laser scanners are mostly pulse-based.
Safety laser scanners have to operate particularly reliable and thus to meet severe safety requirements, for example the EN13849 standard for machine safety and the device standard EN61496 for contactless protective devices (ESPE). In order to meet these safety standards, a number of measures must be taken, such as safe electronic evaluation by redundant, divers electronics, function monitoring or, in particular, monitoring a contamination of optical components such as a front screen.
In DE 20 2013 102 440 U1, a front screen monitoring of a laser scanner is done by a separate transmitter and receiver by means of which monitoring beams are directed onto the front screen and are received after reflection. EP 2 237 065 A1 uses a testing device, which is arranged on a rotor, and a reflector element arranged outside the front screen. EP 1 813 961 B1 proposes, in a camera, to record the reflex on the inside of a front disk with an image sensor, and to evaluate the same for a contamination detection.
In order to obtain reliable data over the entire detection area, there are high requirements not only for the actual measurement of the distances to detected objects, but also for the accuracy of the angle measurement. Conventionally, an encoder disk with regular openings is used for this purpose. A fork light barrier transmits light through the openings of the rotating encoder disk, and a rotational speed or angular position, respectively, is calculated by determining the number of beam interruptions per time. In this case, a relatively large number of components including a rather complex mechanism for the encoder disk and the fork light barrier are used. Individual openings of the encoder disk may be closed during operation by interfering particles or dust, which leads to maintenance requirements or even measurement errors and failure. The forked light barrier is affected by ambient light.
DE 10 2006 041 307 A1 discloses an optoelectronic sensor arrangement, for example a reflective scanning sensor, having an image sensor evaluating successive images in order to detect a relative movement of a detected object. However, this is in no way related to a laser scanner or its scanning movement.
EP 2 626 671 A1 relates to position determination with a scanning unit moving with respect to a surface and having an image sensor. A computing unit determines a travel path or velocity from a difference between successively detected light intensity patterns. This is used for example to determine a position of a vehicle on a transport route.